The present invention relates particularly to the medical imaging art. Specifically, apparatus and method for maintaining the contrast range of a video monitor display within the brightness acceptance range of a photographic recording material are described.
Video imaging apparatus has been developed which permits, through the use of radiology or other techniques in conjunction with computer technology, a safe and accurate view of the interior of the human body. Among these techniques are computerized tomography (CT), positron emission tomography (PET), nuclear magnetic resonance (NMR) and diagnostic ultrasound examination.
All of these techniques depend upon a video display system for presenting the scanned image data to the diagnostician. Such display systems are ultimately resolution limited by imperfections in the phosphor screen of the cathode ray tube (CRT), by the minimum spot size achievable on the video monitor and, in some circumstances, by bandwidth limitations within the beam control circuits.
Additional limitations of CRT display devices are the change in brightness level and/or overall image contrast range which can occur over a period of time. All of these deficiencies result in a loss of image detail when hard copies of the displayed images are produced through photographic techniques. Such hard copies are often needed to enable later diagnosis and comparison between images formed before and after a patient's medication and treatment.
Camera systems for making permanent copies of video monitor images are described in U.S. Pat. Nos. 4,027,315 and 4,240,729, both assigned to the assignee of this application. Such camera systems provide for forming multiple images on a single sheet of photographic material. In accordance with the teachings of the aforementioned patents, the images from a CRT monitor are positioned at different locations on the sheet.
A drift in CRT image brightness can result in a loss of image detail as a result of under or over exposure of the photographic film. In order to preserve the maximum resolution of the system, the absolute brightness and brightness range of the CRT image is advantageously limited in order to avoid exposure errors on the film. Stabilizing the maximum image brightness level at a point just below the shoulder of the film density versus exposure characteristic avoids over exposing the film, with an attendant loss in image detail. A similar requirement exists for stabilizing the brightness of image details to be recorded above the toe region of the film density versus exposure characteristic, in order to maximize the overall resolution of the recorded image.
Of prime interest to a medical diagnostician is detail contained between each of these end points. By maintaining the end points at known brightness levels, detail between the end points is preserved. Any variation of detail within this range of interest raises substantial questions for the diagnostician as to whether a complete and accurate image is available upon which to provide a correct diagnosis. Local contrast in the area of interest is extremely important, and efforts to maintain the local contrast stable with time will ultimately enhance the confidence of the diagnostician in later produced images.
The portion of the density versus exposure characteristic lying between the toe and shoulder regions of a given photographic material, known to those skilled in the art as the brightness acceptance range of the photographic emulsion, should therefore be matched to the absolute brightness and contrast range of the CRT image, and maintained in that matched state over at least the period of time during which photographs of the images are being made. Further, it is desirable to match and maintain this relationship for any of the photographic materials used by those skilled in recording the video images which have different density versus exposure characteristics.